Photofunctional hamburgers

29 June 2007

A hamburger-shaped compound could open the way to a new class of molecular architectures.

A team of researchers led by Takahiko Kojima and Shunichi Fukuzumi at Osaka University, Japan, fused a molybdenum-porphyrin complex and a tungsten polyoxometalate to form a compound they have named the porphyrin hamburger.

Two saddle-shaped porphyrin complexes make up the burger buns, while a cluster of tungsten oxide anions surrounding a central silicon cation, known as a polyoxometalate, forms the meat sandwiched between them. The molecules are joined by stable coordination bonds.

The porphyrin hamburger: tungsten atoms are shown in pink, oxygen in red, molybdenum in green, carbon in gray and nitrogen in blue. The central silicon ion is shown in dark gray.

The structure combines two kinds of photoresponsive, redox-active molecules. This means the molecules can respond chemically to light and can participate in reduction-oxidation processes. For example, polyoxometalates have been used in applications such as catalytic oxidation reactions and optoelectronics, explained Kojima. 'The fusion of these two functional molecules will give rise to novel photofunctional materials for light energy conversion,' he said.

Lee Cronin, an expert in the design of complex molecular architectures at the University of Glasgow, explained how the porphyrins effectively encapsulate the large metal oxide structure in an organic sheath. 'Not only does this give rise to new electronic properties, it raises the prospect of engineering metal oxide structures that can be made bio-compatible or allow catalysis in non-aqueous systems,' he said.

The Japanese team hope to develop their work in both solution and solid phase, looking in particular at catalytic oxidation in solution and photoinduced proton conduction in the solid state.